FIG. 1 is a diagram illustrating a wireless communication system. Referring to FIG. 1, a wireless communication system 100 includes a plurality of base stations 110 and a plurality of mobile stations 120. The wireless communication system 100 can include homogeneous networks or heterogeneous networks. In this case, heterogeneous networks refer to networks in which different network entities coexist, such as IEEE (Institute of Electrical and Electronics Engineers) 802.16e system, IEEE 802.16m system, 3GPP LTE (Long Term Evolution) system, and WIFI system. Also, the homogeneous networks can be classified from a macro cell, a femto cell, a pico cell, and a relay station. The base stations are fixed stations that perform communication with mobile stations. Each of the base stations 110a, 110b, and 110c provides services to specific geographical area 102a, 102b or 102c. In order to improve system throughput, the specific area may further be divided into a plurality of smaller areas 104a, 104b and 104c. These smaller areas may be called cells, sectors or segments. In the IEEE 802.16 system, cell Identity (ID) (Cell_ID or IDCell) is assigned based on the whole system, whereas sector ID or segment ID is assigned based on a specific area within the range of service coverage of each base station, and has a value from 0 to 2. The mobile station 120, which is mobile or fixed, is generally distributed over the wireless communication system. Each mobile station may perform communication with at least one base station on a downlink and an uplink at a point of time. The communication may be performed between the base station and the mobile station through Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Single Carrier-FDMA (SC-FDMA), Multi Carrier-FDMA (MC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or a combination of them. In this specification, a downlink refers to a communication link directed from a base station to the mobile station and an uplink refers to a communication link directed from the mobile station to the base station.